This invention relates to the deposition of hydrogenated amorphous silicon films or certain alloys thereof on substrates and, in particular, to an improved method and apparatus for control thereof.
Photovoltaic solar cells made of thin film semiconductor layers, such as hydrogenated amorphous silicon (a-Si:H) may be made in a sandwich structure consisting of p i n layers. Such structures are well known in the art and typically the p type layer is boron-doped and of a thickness of about 10 nm, the i type, or intrinsic, layer is undoped and of a thickness of about 500 nm, and the n type layer is phosphorus-doped having a thickness of about 10-30 nm. The electron/hole pairs are generated in the i layer and collected by virtue of the built-in electric field which results due to the junctions formed by the p and n layers with the i layer. When i layers are grown they may ultimately be characterized as slightly n type (n.sup.-) or slightly p type (p.sup.-) depending on the background impurities caused by other materials being present within the chamber during the deposition process. It is existing practice in the art to grow the i layer without strictly controlling the characteristic p.sup.- or n.sup.- that the i layer may take. The reason for this is that at the present time, there are no known methods for measuring such low levels of impurities, of the order of 10.sup.-6, in a glow discharge atmosphere during deposition. The resulting type of the i layer (p.sup.- or n.sup.-) can have a substantial influence on the conversion efficiency of a particular solar cell structure.
The present invention permits precise control of the characteristic p.sup.- or n.sup.- that the i layer may take by the novel use of the photovoltaic properties of the outer surface of the i layer being grown. That is, since only a very thin outer layer of a-Si:H will absorb a substantial amount of light of a particular frequency a photovoltage is thereby generated at its surface which is indicative of it characteristic p.sup.- or n.sup.- type.